I like a girl what to do??

i'm a female and i'm 19 years old i'm also straight well i thought i was up until now, there is this girl that goes to my school and she is a she's a butch(dresses like a dude) i never paid her any attention up until last week i just looked at her like "wow she is really pretty" and left it alone but the past couple of days she been on my mind like crazy, and i don't know what to do i want to tell her but she has a girlfriend, and i'm feeling her so0o0o0o much it's driving me crazy, she is the first girl i ever looked at this way so i'm nervous and i don't know if it's just a phase or not people please help me and give me some advice on what to do